U.S. patent number 4,560,922 [Application Number 06/665,514] was granted by the patent office on 1985-12-24 for method for determining the direction of the origin of a disturbance affecting an element of an electrical energy transfer network.
This patent grant is currently assigned to Enertec. Invention is credited to Isabelle Heller, Paul Loewenstein.
United States Patent |
4,560,922 |
Heller , et al. |
December 24, 1985 |
Method for determining the direction of the origin of a disturbance
affecting an element of an electrical energy transfer network
Abstract
A directional relay for providing indication of the direction of
a fault on an electric power transfer network has an adder (9), a
subtractor (10) and filters (11) which receive signals from voltage
and current transformers (6,7) and derive signals related to
fault-induced forward and backward transient waves on the network.
To ensure reliable operation even when the relay is close to a low
impedance element such as a generator (2), the forward wave related
signal is delayed in a delay unit (12) by an amount equal to the
propagation time of the wave along the section protected by the
relay, before the difference between its square and that of the
backward wave related signal are integrated (16) to derive the
fault direction signal. If the protected section has a low
distributed capacitance, the voltage and current signals can
instead be combined by a differentiator (22) and adder (23) and
filtered (11B) to produce a signal related to the forward transient
wave appearing at a point X along the protected section. The
integrator (16) receives the product of this signal and the
filtered current signal.
Inventors: |
Heller; Isabelle (Sevres,
FR), Loewenstein; Paul (Mountain View, CA) |
Assignee: |
Enertec (Montrouge,
FR)
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Family
ID: |
9248306 |
Appl.
No.: |
06/665,514 |
Filed: |
October 31, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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324247 |
Nov 23, 1981 |
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Foreign Application Priority Data
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Nov 25, 1980 [FR] |
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80 24946 |
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Current U.S.
Class: |
324/522;
324/521 |
Current CPC
Class: |
H02H
7/265 (20130101); G01R 31/085 (20130101) |
Current International
Class: |
G01R
31/08 (20060101); H02H 7/26 (20060101); G01R
031/08 (); H02H 003/38 () |
Field of
Search: |
;324/51,52,58.5A,58.5B
;361/82 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Krawczewicz; Stanley T.
Assistant Examiner: Solis; Jose M.
Attorney, Agent or Firm: Gaudier; Dale
Parent Case Text
This application is a continuation of application Ser. No. 324,247,
filed Nov. 23, 1981, now abandoned.
Claims
We claim:
1. A method for determining, at a measuring point situated at one
end of a protected section of an electricity transmission line, the
direction relative to that point of the origin of a disturbance
affecting the protected section, comprising the steps of:
(a) measuring the voltage and current of a signal arising as a
result of said disturbance at the measuring point and developing
voltage and current responsive signals;
(b) processing by filtering, delaying by a value proportional to
the length of the protected section and squaring the voltage and
current responsive signals to obtain a first signal;
(c) processing by filtering and squaring the voltage and current
responsive signals to obtain a second signal; and
(d) combining the first signal and the second signal to produce an
output signal having a value representative of the integral of the
instantaneous difference between these signals and an algebraic
sign indicative of the direction of said disturbance relative to
the measuring point.
2. The method of claim 1 wherein the first signal formed in step
(b) has the form of (V+KI).sup.2 and the second signal formed in
step (c) has the form of (V-KI).sup.2, where V and I are,
respectively, the voltage and current measured in step (a) and K is
a constant.
3. The method of claim 2 wherein K is substantially equal to the
characteristic impedance of the transmission line.
4. The method of claim 1 further including the steps of:
comparing a value of the signal produced in step (d) with a
preselected threshold indicative of the residual energy on the
transmission line when no disturbance is present; and
indicating that a disturbance is present when the value of this
signal exceeds the preselected threshold.
5. The method of claim 1 wherein in step (b) the delay is
substantially equal to the propagation time of a transient power
wave over the protected section.
6. A method for determining, at a measuring point situated at one
end of a protected section of an electricity transmission line, the
direction relative to that point of the origin of a disturbance
affecting the protected section, comprising the steps of:
(a) measuring the voltage and current of a signal arising as a
result of said disturbance at the measuring point and developing
voltage and current responsive signals;
(b) processing by summing and filtering the voltage and current
responsive signals to obtain a first signal;
(c) filtering the current responsive signal;
(d) multiplying together the first signal and the filtered current
responsive signal to obtain a second signal; and
(e) integrating said second signal to produce an output signal
having an algebraic sign indicative of the direction of said
disturbance relative to the measuring point.
7. The method of claim 6 wherein the known characteristics of the
protected section comprise inductance and impedance.
8. The method of claim 7 wherein the first signal of step (b) is
equal to V=RI-L.multidot.dI/dt, where V and I are, respectively,
the voltage and current measured in step (a), R is the
characteristic impedance of the protected section, and L is the
characteristic inductance of the protected section.
9. The method of claim 6 wherein the point spaced from the
measuring point is located at substantially the mid-point of the
protected section.
10. The method of claim 6 wherein the electricity transmission line
has negligible distributed capacitance.
11. A method for determining, at a measuring point situated at one
end of a protected section of an electricity transmission line, the
direction relative to that point of the origin of a disturbance
affecting the protected section, comprising the steps of:
(a) measuring the voltage V and current I of a signal arising as a
result of said disturbance at the measuring point and developing
voltage and current responsive signals;
(b) processing the voltage and current responsive signals to obtain
a first signal having a form (V+KI).sup.2, wherein K is
substantially equal to the characteristic impedance of the
transmission line;
(c) imparting to the first signal a delay proportional to the
length of the protected section;
(d) processing the voltage and current responsive signals to obtain
a second signal having the form (V+KI).sup.2 ; and
(e) processing the delayed first signal and the second signal to
produce an output signal having an algebraic sign indicative of the
direction of said disturbance relative to the measuring point.
Description
This invention relates to a method for determining, at a measuring
point situated at one end of a protected section of a part, such as
an electricity transmission line, of an electrical energy transfer
network, the direction, relative to that point, which indicates the
origin of a disturbance affecting that part, comprising the steps
of: forming, from signals indicative of the voltage and current at
the measuring point, a quantity having the algebraic sign of the
integral of the difference between a forward transient power wave
passing the measuring point and proceeding from outside the
protected section towards that section and a backward transient
power wave passing the measuring point and proceeding from the
protected section towards the outside, the said integral
representing the total transient energy passing the measuring
point; and deriving the said direction from the algebraic sign of
the said quantity.
It is known that electrical energy transfer networks experience
various electrical disturbances, such as connection or
disconnection of generators or loads, the most serious of these
disturbances, called faults, involving for example short circuits
from one or several of the conductors of a line to earth, or, in
the case of a polyphase network, between the conductors allocated
to different phases in the same line.
To avoid the damaging effects of faults, electrical energy transfer
networks are provided with detectors and complex control and
command devices arranged to ensure the protection of the faulty
part of the network, for example the line or the transformer, in
the shortest possible time after the appearance of the fault, by
disconnection of this part from the circuit.
Bearing in mind that each detector makes instantaneous measurements
of voltage and current at a given point on the part of the network,
it is necessary at this measuring point not only to be able to
determine the existence of a fault on the part of the network but
equally to be able to determine on which side of this point the
fault is located.
It has been long known that the appearance of a fault on part of a
network involves a transient phase change the polarity of which
indicates the direction of movement and thus the direction of the
origin of the fault with reference to the measuring point.
Conventionally two detectors, known as directional relays, are
used, each relay being located at the opposite ends of a section to
be protected of a part of an electrical energy transfer network.
This section is usually chosen so as not to include any equipment
likely to cause disturbances in normal operation, and each relay is
arranged to detect whether or not a disturbance comes from the
protected section. To this end each relay monitors, during a
certain time at least after the start of every disturbance
detected, the sign of the total transient energy passing the
measuring point, indicated for example by the similarity or
dissimilarity of the signs of the voltage and the intensity of the
transient wave. For each disturbance, the directions detected by
the relays are compared, for example by radio communication. If
both relays indicate that the disturbance came from the protected
section, steps are immediately taken to protect the part of the
network concerned; otherwise, the disturbance is considered as
being external to the protected section. An example of such a prior
art system is shown in U.S. Pat. No. 4,251,766.
A difficulty arises in the case where the impedance upstream of the
relay is low, for example, for an electricity transmission line, in
the case where the only significant impedance outside the protected
section is that of a large electrical generator connected to the
line close to the relay.
Considering, theoretically, the total transient energy passing the
measuring point as the integral with respect to time of the
difference between a forward transient power wave, proceeding from
outside the protected section, or upstream, towards that section,
and a backward transient power wave, proceeding from the protected
section towards the outside, it appears that the detection of the
direction of the fault is all the more difficult since the forward
and backward waves are similar in amplitude and close in time, the
reason being that the sign of a positive or negative physical
quantity very close to zero is fraught with uncertainty. Hence, in
the case of a fault affecting the protected section and the
presence of a low impedance upstream of the directional relay, the
forward wave is formed by the reflection of the backward wave
upstream of the relay, and these two waves have very similar
amplitudes, by virtue of the almost complete absence of
attenuation, and are very close in time.
There is as a result, in the determination of the direction of the
origin of a fault with respect to the measuring point, a risk of
error which the present invention is intended to alleviate.
The method of this invention, which comprises in known manner the
operations of forming a quantity the algebraic sign of which is
that of the total transient energy passing the measuring point and
of deriving from the algebraic sign of this quantity the direction
of the origin of the disturbance, is essentially characterised in
that the said quantity has an absolute value substantially equal,
apart from a constant of multiplication, to the integral of the
instantaneous difference between the said forward transient power
wave, delayed by a period of time between zero and twice the
propagation time of a transient power wave over the protected
section, and the said backward transient power wave.
It is to be understood, in the foregoing and the rest of this
description, that expressions such as `power wave`, `difference of
power waves` and `integral of the difference of power waves` can
obviously mean the physical quantities thus named, but equally, for
the sake of brevity, signals representative of these physical
quantities.
In the case where the part of the network is a line having
negligible distributed capacitance, the forward transient power
wave can be obtained at any instant by deriving, from the forward
transient power wave passing the measuring point at that instant
and the characteristics of the line, the fraction of the forward
transient power wave which, having passed the measuring point, will
pass or at least ought to pass, at a later instant, a point on the
protected section spaced from the measuring point, for example at
the mid-point of the protected section.
Other characteristics and advantages of the invention will emerge
clearly from the description thereof which follows, by way of
non-limitative example, with reference to the accompanying
drawings, in which:
FIG. 1 is a schematic diagram representing a conductor of an
electrical energy transmission line, with a section protected by
two directional relays, a part of low impedance outside the
protected section and a fault also outside the protected
section;
FIG. 2 is a schematic diagram representing a conductor of an
electrical energy transmission line, with a section protected by
two directional relays, a part of low impedance outside the
protected section, and a fault within this section;
FIGS. 3 and 4 are diagrams corresponding respectively to FIGS. 1
and 2 and each showing on the positive ordinate, in terms of energy
E as a function of time t, the integral of the transient power wave
proceeding from outside the protected section towards that section
(forward wave), and on the negative ordinate the integral of the
transient power wave proceeding in the opposite direction (backward
wave);
FIG. 5 is a circuit diagram of an apparatus for implementing the
method of the invention; and
FIG. 6 is a circuit diagram of another apparatus for implementing
the method of the invention, applicable to the case of a line
having negligible distributed capacitance.
FIGS. 1 and 2 show a conductor 1 of an electrical energy
transmission line, an electrical energy generator 2 connected to
the line, and two directional relays 3 and 4 protecting the section
of the line located between them, the generator 2 having a low
impedance and constituting the primary impedance upstream of the
relay 3.
The conductor 1 is used to carry one phase in the case of a
polyphase current, and the generator 2 is outside the protected
section, close to the directional relay 3.
If a fault 5 occurs on the line between the generator 2 and the
relay 3, a transient power wave A1 appears, called `forward`
because it is directed towards the protected section (arrows in
full line in FIG. 1), the integral of which with respect to time
has for example the form JA1 shown in FIG. 3, likewise in full
line. This `forward` wave is reflected beyond the other directional
relay 4, and forms a backward wave R1 having a corresponding
integral JR1 (in dashed line in FIGS. 1 and 3).
At the relay 3, the backward wave has, relative to the forward
wave, on the one hand a significantly attenuated amplitude by
virtue of the loss of energy which occurs upon reflection and
during propagation of the wave over at least twice the length of
the protected section, and on the other hand a significant phase
shift, corresponding to the propagation time over at least twice
the length of the protected section.
This attenuation and time shift, which appear in the integrals JA1
and JR1 of these power waves, are clearly shown in FIG. 3, in which
the integrals of the power waves are shown with arbitrary forms and
in which the energies JA1 and JR1 are plotted along positive and
negative ordinates respectively.
If SJR1 denotes the mirror-image curve of JR1 about the axis of the
abscissa t, the total transient energy which is already dissipated
for one phase at time t is given by the difference, positive in
sign, between the ordinate of JA1 and that of SJR1 at that time t.
FIG. 3 shows clearly that this difference has a relatively large
absolute value, such that the sign of this difference can be
readily identified.
In contrast, referring now to FIGS. 2 and 4, showing the case where
a fault 5 occurs on the protected section, it can be seen that the
situation is substantially different.
The directional relay receives first the backward wave R2, to which
there corresponds the integral JR2 (in full line in FIGS. 2 and 4),
and this wave is reflected by the low impedance 2 to form the
forward wave A2, to which there corresponds the integral JA2 (in
dashed line in FIGS. 2 and 4). But, bearing in mind that the
reflection is then almost total and that the path from the relay 3
to the generator 2 is short, the forward wave passes the relay 3
very shortly after the backward wave and with substantially the
same amplitude.
This effect which appears in the integrals JA2 and JR2 of the
forward and backward power waves, is illustrated in FIG. 4.
The curves shown in this figure have arbitrary forms and the
energies JA2 and JR2 are plotted along positive and negative
ordinates respectively.
If SJA2 denotes the mirror-image curve of JA2 about the axis of the
abscissa t, the transient energy which is already dissipated for
one phase at time t is given by the difference, negative in sign,
between the ordinate of JR2 and that of SJA2 at that time t. FIG. 4
shows clearly that this difference has a small absolute value, such
that the determination of the algebraic sign of this difference
involves a significant risk of error.
The method of the invention consists essentially of delaying the
signal representing the forward power wve A2 (in dashed line in
FIG. 2) by a period of time between zero and twice the propagation
time of a transient power wave along the length of the protected
section; preferably, this period of time is chosen equal to this
propagation time itself.
The power wave of which this delayed signal is representative has a
corresponding time integral or energy JA'2, delayed relative to
JA2. If SJA'2 denotes the mirror-image curve of JA'2 about the time
axis, the quantity whose algebraic sign is considered to determine
the direction of the origin of the fault is, according to the
invention, the difference between the corresponding ordinates of
the curves JR2 and SJA'2 or any quantity proportional to this
difference. As can be seen from FIG. 4 this difference has an
absolute value larger than that of the difference between the
ordinates of JR2 and SJA2, so the sign can be determined with
greater reliability.
It must of course be noted that the delay effectively introduced in
the forward wave is also responsible (FIG. 3) for a displacement of
the curve JA1 to the curve JA'1 in the case of a fault outside the
protected section. This displacement does not however alter the
algebraic sign of the difference between the delayed forward wave
and the backward wave, since it remains less than the propagation
time of a transient power wave along twice the length of the
protected section. In fact, since JA'1 is not (FIG. 3) displaced to
the right of SJR1, the quantity whose sign is monitored, namely the
difference between the corresponding ordinates of JA'1 and SJR1,
preserves the same sign. Nonetheless, in order that the absolute
value of this difference should be sufficiently large that the
determination of its sign should be reliable, it is preferable that
the delay introduced is equal or close to the propagation time of a
power wave along the length of the protected section, JA'1 then
being substantially midway between JA1 and SJR1.
FIG. 5 shows an apparatus arranged to implement the method of the
invention in the general case.
This apparatus incorporates, in conventional manner, a current
transformer 6 in series with the conductor 1 of the line and a
voltage transformer 7 in shunt, and comprises a multiplier 8 for
multiplying the output signal I of the current transformer 6 by a
predetermined parameter K, an adder 9 and a subtractor 10 for
forming respectively the signals V+KI and V-KI, V being the output
signal of the voltage transformer 7, filters 11A and 11B for
removing from the signals V+KI and V-KI the normal operating
frequency of the energy transmission or distribution network, a
delay unit 12 for delaying the signal V+KI for example after
filtering, function generators 13A and 13B for forming the signals
(V+KI).sup.2 r and (V-KI).sup.2 respectively, the suffix r
indicating the introduction of a delay, a subtractor 14 for forming
the signal [(V+KI).sup.2 r-(V-KI).sup.2 ], an integrator 16, having
an input resistance 15, for forming the signal
J=.intg.[(V+KI).sup.2 r-(V-KI).sup.2 ].dt, and a logic circuit 17,
comprising two comparators 18 and 19, receiving the signal J on
their negative and positive inputs respectively, and a threshold
signal S on their positive and negative inputs respectively.
The signal S is provided to allow for the residual value of the
signal J in the absence of a fault on the conductor 1. It has been
mentioned several times above that the polarity of the total
transient energy propagating on the conductor 1 allocated to the
transfer of one phase in a polyphase network was, in the case of a
fault on that phase, indicative of the direction of the origin of
the fault. All the same transient energy also appears, in case of a
fault, on the phases which are not faulty, but the polarity of this
energy does not provide any indication of the direction of the
origin of the fault. It is thus necessary to identify the faulty
phase or phases to ensure the validity of the indication of the
direction of the origin of the fault. Bearing in mind that the
total transient energy appearing on a faulty phase is larger than
that which appears on a phase unaffected by the fault, the use of a
threshold signal S enables positive activation of one of the
outputs 20 and 21 of the comparators 18 and 19 to be obtained only
if the fault affects the phase monitored by the transformers 6 and
7. Each output 20 or 21 which is positively activated corresponds
to one direction to the origin of the fault relative to the
measuring point M where the transformers 6 and 7 are located.
The parameter K is set at a value equal to that of the
characteristic impedance of the line, so that the signals V+KI and
V-KI represent voltages the squares of which are proportional to
powers corresponding to the forward and backward waves
respectively.
Those skilled in the art will understand, from reading the above
description, that the filters 11A, 11B and the delay unit 12 can be
disposed elsewhere in the circuit described, the filter 11A and the
unit 12 for example being interchanged.
FIG. 6 shows an apparatus for implementing the method of the
invention in the particular case of a short line, that is to say
having a low distributed capacitance, or more precisely a line the
quarter-wave frequency of which is much larger than the bandwidth
of the signal used for the determination of the direction of the
origin of the fault.
This apparatus includes, besides the parts 11A, 11B, and 15 to 21
described with reference to FIG. 5, a differentiator 22, an adder
23 and a multiplier 24.
The differentiator 22 receives the signal I from the current
transformer 6 and produces on its output 25 the signal ##EQU1##
where R and L are respectively the equivalent resistance and series
inductance of a given length of the line, preferably of half the
length of the protected section of this line. The adder 23 is
coupled at its inputs to the differentiator 22 and to the voltage
transformer 7, so that it supplies at its output a signal
##EQU2##
This signal U, with the dimensions of a voltage, represents what
subsequently will be, or at least would be for a fault-free line on
which the same transient power waves were propagating, the voltage,
a fraction of which is associated with the forward wave having
passed the measuring point M and appearing at a point X on the line
spaced from the point M and preferably chosen to be at the
mid-point of the protected section.
The filters 11A and 11B eliminate from the signals I and U
respectively the normal operating frequency of the network, and the
filtered signals I and U are multiplied by the multiplier 24 the
output of which supplies the arrangement 15 to 21 described above.
The distributed capacitance of the line being negligible, the
strength of the transient wave which could have been measured at
the point X on the protected section is comparable to the signal I,
so the integrator 16 provides a signal representative of the
integral of the difference between a forward transient power wave
and a backward transient power wave appearing at the same time at
different points on the line or, equivalently, at the same point on
the line but at different instants, the forward wave being delayed
relative to the backward wave.
For the sake of clarity, the above description refers to the
detection of disturbances affecting electrical energy transmission
lines. Nevertheless the same techniques are applicable to the
protection of other parts of electrical energy networks, for
example transformers.
Furthermore, although the examples given make use of analogue
techniques, digital techniques can also be used. Thus, for example,
referring to FIG. 5, analogue to digital converters could be
connected to the outputs of the transformers 6 and 7, and the
functions of the parts 8 to 19 could be implemented by one or
several computers.
* * * * *